Display devices are known wherein characters, cursor, lines, etc. are displayed by illuminated pixels (also known as dots, pels or picture elements) with a black or non-illuminated background on a display screen. The display operations for displaying characters, cursor, etc. by the illuminated pixels on a black background will be referred to as a negative display mode or a white on black mode. The negative display mode includes a normal mode and a high intensity mode. In the normal mode characters, cursor, etc. are displayed with low luminance pixels on a black or non-illuminated background. In the high intensity mode the characters, cursor, etc. are displayed with high luminance pixels on a black or non-illuminated background. To control the luminance of a pixel, two binary bits are assigned for each pixel, and represent the luminance of one pixel, as follows:
0 0 . . . Zero luminance (Black) PA1 0 1 . . . Low luminance (Gray) PA1 1 0 . . . High luminance (White) PA1 1 1 . . . Not used
Pixel patterns generated from a font memory include foreground pixels representing the characters to be displayed in combination with background pixels.
Japanese patent application 47-98526 (Published examined patent application No. 54-3581) shows a CRT display device wherein the background is displayed by low luminance, negative value is displayed by zero luminance, and positive value is displayed by high luminance. As a result, the negative and positive values in an equation are distinguishably displayed on the display screen. The application 47-98526, however, does not disclose the concept of the invention.